A CODEC is a device comprised of an encoder for encoding signals and a decoder for decoding signals. In practice, the encoder portion of the CODEC comprises an Analog-to-Digital (A/D) converter, and the decoder portion comprises a Digital- to-Analog (D/A) converter. CODECs are commonly used in telephony for encoding and decoding signals passing between a digital multiplexing switching and transmission system, and a subscriber loop serving a plurality of analog voice terminals.
For proper operation, the CODEC must perform signal encoding and signal decoding. Thus, complete testing a CODEC requires that operation of both the encoder and decoder portions be verified. Presently, external test equipment is often used to test the CODEC by applying an analog and a digital signal to the encoder and decoder portions, respectively, of the CODEC, causing the encoder and decoder to generate a digital code and an analog signal, respectively. The digital code and analog signal generated by the encoder and decoder, respectively, are then analyzed by the external test equipment to determine whether each portion of the CODEC was operating properly. The disadvantage of this approach is that the external test equipment employed to test the CODEC is expensive and the test time is long.
Another approach to testing a CODEC is disclosed in the paper "A Built-In Self-Test for ADC and DAC in a Single-Chip Speech CODEC" by E. Teraoka et at., presented at the International Test Conference '93 (Baltimore, Md.) on Oct. 21, 1993. In their paper, Teraoka et al. accomplish self-testing of a CODEC containing both an A/D converter and a D/A converter, as well as a Digital Signal Processor (DSP), by first down-loading blocks of digital information, representing a sinusoidal waveform, to the input of the D/A converter. The output of the D/A converter is coupled to the input of the A/D converter of the CODEC so that the A/D converter, when operating properly, should produce a digital signal corresponding to the digital signal input to the D/A converter of the CODEC. The output of the A/D converter is then analyzed by the DSP, using Fourier Transform analysis, to detect missing codes. While this scheme eliminates the need for external test equipment, the scheme is only effective if the CODEC contains a DSP. Unfortunately, not all CODECs contain such a DSP and adding a DSP simply for testing purposes would be impractical as well as expensive.
Thus, there is a need for a built-in self-test technique for testing a CODEC which overcomes the disadvantages of the prior art.